UITextField deleteBackward() Not Called When Custom Keyboard is In Use

Number:rdar://FB7707493 Date Originated:
Status: Resolved:
Product:iOS Product Version:
Classification: Reproducible:
If you subclass UITextField in order to listen for deleteBackward() on UIKeyInput this only works with the native keyboard. I've tried various custom keyboard extensions and none of them trigger deleteBackwards when deleteBackwards is pressed on the keyboard. 

To recreate try this:

class MyTextFieldSubclass: UITextField {
    override func deleteBackward() {
          // This should be called on delete backwards but it's not.

Note. I've only tested it with 
keyboardType = .numberPad


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